Modern electronic devices such as a notebook computer comprise a variety of memories to store information. Memory circuits include two major categories. One is volatile memories; the other is non-volatile memories. Volatile memories include random access memory (RAM), which can be further divided into two sub-categories, static random access memory (SRAM) and dynamic random access memory (DRAM). Both SRAM and DRAM are volatile because they will lose the information they store when they are not powered. On the other hand, non-volatile memories can keep data stored on them permanently unless an electrical charge is applied to non-volatile memories. Non-volatile memories include a variety of sub-categories, such as electrically erasable programmable read-only memory (EEPROM) and flash memory.
A flash memory comprises a plurality of memory cells arranged in rows and columns. Each memory cell stores a bit of information by means of a floating gate transistor. A sense amplifier is used to verify the logic state of a memory cell by comparing a memory cell current with a reference current. More particularly, a voltage potential is applied to the control gate of the memory cell and then a current flows through the memory cell in response to the voltage change at the control gate. The sense amplifier detects the current flowing through the memory cell and compares it with a predetermined reference current. When the memory cell current is more than the reference current, the sense amplifier reports a logic high, which means the memory cell stores a logic high state. In contrast, when the memory cell current is less than the reference current, the sense amplifier reports a logic low, which reflects the logic low state of the memory cell.
Sense amplifiers include voltage mode sense amplifiers and current mode sense amplifiers. In a voltage mode sense amplifier, both a memory cell current and a reference current are converted into the corresponding voltages and further fed into the inputs of the voltage mode sense amplifier. The READ operation of the memory cell is performed by comparing the two voltages at the inputs of the voltage mode sense amplifier. The voltage mode sense amplifier reports the logic state of the memory cell based upon the comparison result.
In contrast, a current mode sense amplifier directly compares a memory cell current with a reference current. In a READ operation of a memory cell, a complementary metal-oxide-semiconductor (CMOS) inverter may be used to monitor the difference between the memory cell current and the reference current. More particularly, the inverter's input is coupled to both the reference current and the memory cell current. Furthermore, the difference between the reference current and the memory cell current is used to charge the input of the inverter. In accordance with the operation of flash memory circuits, when a memory cell stores a logic low state, the reference current is more than the memory cell current. As a result, the difference between the reference current and the memory cell current is a positive value, which means a charge current charges the input of the inverter to a level more than the trigger point of the inverter so that the current mode sense amplifier reports a logic low state. In contrast, when the memory cell stores a logic high state, the reference current is less than the memory cell current. As a result, the difference between the reference current and the memory cell current is a negative value, which means a discharge current discharging the input of the inverter down to a level below the trigger point of the inverter so that the current mode sense amplifier reports a logic high state.
In a READ operation, the access time of detecting a logic state stored in a memory cell is a key performance index for a memory circuit. A sense amplifier having a shorter sense time may cut the total access time so that the performance of the memory circuit is improved. Moreover, the power consumption of a memory circuit has become one of major challenges in designing high performance memory circuits. As a result, a sense amplifier consuming less power may contribute to the total power saving of a memory circuit.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.